1. Field of the Invention
The present general inventive concept relates to an audio reproducing system, and more particularly, to a method and apparatus to audio matrix encode/decode, which encode and decode audio signals of two or more channels into an audio signal of one or more channel while preserving a direction of a sound image.
2. Description of the Related Art
While viewers, conventionally, could watch movies or programs through terrestrial television broadcasting, recent distribution of video tapes, video discs, and satellite broadcasting allows the viewers to enjoy original sound of the programs the viewers are watching. For such the original sound that is available by the video tapes, video discs, and satellite broadcasting, audio signals of a plurality of channels are encoded into audio signals of two channels by performing matrix process. The audio signals of two channels which are encoded by the matrix process can be reproduced as stereo sounds. Also, by using a particular decoder, audio signals of five channels including a front left channel L, a center channel C, a front right channel R, a left surround channel Ls, and a right surround channel Rs can be restored from audio signals of two channels. From among the audio signals of five channels, the center channel signal functions to achieve localization of the sound, which is involved with an articulation of the sound and the surround channel signals function to increase a realistic impression of the sound by moving sounds, surround sounds, and reverberation sounds.
The conventional matrix decoder creates a center channel signal and surround channel signals using addition and subtraction of signals of two channels. An audio matrix in which matrix characteristics are most changed is known as a passive matrix decoder. In each channel signal separated by the passive matrix decoder, when encoding is performed, other channel audio signals are scaled down and linearly combined together. Thus, the signals of channels output by the conventional passive matrix decoder has low channel separation, and thus the localization of the sound image is not precisely defined. An active matrix decoder adaptively alters matrix characteristics in order to increase the separation of two-channel matrix-encode signals.
U.S. Pat. No. 4,799,260 (filed on 6 Feb. 1986 entitled “Variable Matrix Decoder”) and WO 02/19768 A2 (filed on 31 Aug. 2000 entitled “Method for Apparatus for Audio Matrix Decoding), relates to a matrix decoder.
FIG. 1 is a block diagram illustrating a matrix decoder according to the conventional art. Referring to FIG. 1, in the conventional matrix decoder, gain functions 110 and 116 clip input signals in order to keep balance between levels of stereo signals Lt and Rt. A passive matrix function 120 outputs passive matrix signals from stereo signals L′t and R′t output from the gain functions 110 and 116. A variable gain signal generator function 130 generates six control signals gL, gR, gF, gB, gLB, and gRB in response to the passive matrix signals generated by the passive matrix function 120. A matrix coefficient generator function 132 generates twelve matrix coefficients in response to the six control signals generated by the variable gain signal generator function 130. An adaptive matrix function 114 generates output signals L, C, R, Ls, Bs, and Rs in response to the input stereo signals L′t and R′t and the matrix coefficient generated by the matrix coefficient generator function 132. The variable gain signal generator function 130 monitors the level of the signal of each channel, and calculates optimum linear coefficient according to the monitored level of the signal of each channel in order to reconstruct audio signals of multi channels. The matrix coefficient generator function 132 increases the level of the channel, which has the greatest level, in nonlinear fashion.
However, the conventional matrix decoding system as in FIG. 1 has a difficulty to accurately represent the changes in location of a sound source that moves in a virtual space, thereby, disadvantageously, unable to represent the sound image dynamically. That is, most of reproduced sound energies are output mainly from the front channels (L, R, and C channels), and hence, when signals that have already been down-mixed are up-mixed again, the channel separation of the signals is reduced and movement of the sound image cannot be satisfactorily restored.